1) Field of the Invention
This invention relates to integrated circuits and semiconductor devices. It relates particularly to a structure and method for producing integrated circuits having an improved contamination passivation film for surface features, such as fusible links and photo alignments marks.
2) Description of the Prior Art
Fusible conductive links (fuses) are often used in rewiring electrical circuits to replace defective devices with redundant devices. These circuits are rewired by rendering the fuses non-conductive (i.e., blown) by applying laser energy to the fuse with a laser trimming machine.
In dynamic or static memory chips, defective memory cells may be replaced by blowing the fuses associated with the defective cells, and activating a spare row or column of cells. This circuit rewiring using fusible links considerably enhances yields and reduces the production costs.
Logic circuits may also be repaired or reconfigured by blowing fuses. For example, it is common to initially fabricate a generic logic chip having a large number of interconnected logic gates. Then in a final processing step, the chip is customized to perform a desired logic function by disconnecting the unnecessary logic elements by blowing the fuses that connect them to the desired circuitry. Still other applications of laser-blown fuses are possible.
Semiconductor chips often have openings through protective insulating layers over fusible link regions to allow a laser to irradiate the fuse. But these openings frequently lowers chip yields and reliability by allowing contamination to penetrate from the openings to the device regions.
FIG. 1 shows a top plan view of a semiconductor chip 12 with openings 28 over fusible links surrounded by active device areas 14. A conventional fusible link (i.e., fuse) 26, a fusible link opening 28 and an adjacent device region 60, 61, 62, 66, is shown in top plan view FIG. 2. Referring to FIG. 3, a cross-sectional view of the same link and device regions in FIG. 2 taken along horizontal axis labeled 3 is shown.
An important challenge is to improve the reliability of the semiconductor devices surrounding fusible links, especially when a large number of the fusible link openings 28 are on a chip 12. A window opening 28 is normally formed through the insulating layers over a fuse 26 so that a laser beam can be used to irradiate the fuse, thereby blowing it. A problem with openings 28 over a fusible link regions is that moisture and other contaminates can diffuse from the openings, through the insulating layers, into the semiconductor devices thus reducing circuit reliability and yields.
As shown in FIG. 3, fuse 26 is normally formed over thick field oxide regions 31 in a semiconductor substrate 10. Fuse 26 is formed over the field oxide regions 31 to prevent shorting of the fuse 26 to the substrate 10. Fuse 26 can be formed of a metal, such as aluminum, platinum silicide, titanium tungsten, polysilicon, or a polycide, such as titanium polycide, tungsten polycide or molybdenumpolycide. Layers 32, 34, 36, 38 are insulating (passivation or dielectric) layers, such as borophosphosilicate glass (BPSG), spin on glass, silicon oxide and silicon nitride respectively. Opening 28 is formed over the fuse 26 area through the insulating layers 32, 34, 36, 38. Opening 28 can have a width of 5 microns and a length of 5 microns. An adjacent semiconductor device is shown with buried N+ regions 60, 61, 62, gate oxide 64, gate 66, via 40 and metal layers 68, 70.
A laser is used to blow the fuses 26. The laser is focused through the fuse opening 28 and irradiates the fuse 26. It is conventional to have an opening 28 over the fuse 26 in the area where the fuse 26 will be broken so that the laser heating will be more effective. Because the passivation layers overlying a fuse 26 would reduce the laser energy striking the fuse, the passivation layers are etched away so that the fuse 26 is exposed or, in an alternative, only single thin insulating layer or a portion of a thin insulating layer covers the fuse.
The fuse 26 absorbs the heat from the laser irradiation and the fuse melts. In this operation, sometimes called laser trimming, the rapid temperature rise of the upper portion of the fuse 26 causes an increase in pressure. This pressure cause any overlying film 32 to be "blown off" and the melted polysilicon fuse is removed by evaporation. Laser trimming requires that only a very thin insulating layer cover the fuse because the laser must be able to penetrate the layer and melt the fuse. The portion of the fuse 26 and thin insulating layer 32 over the fuse which is melted away or "blown" must not deposit on or interfere with near-by devices. In the example shown in FIG. 3, an opening 28 is formed through four layers: Silicon nitride 38, silicon oxide 36, spin on glass 34 and borophosphosilicate glass (BPSG) layer 32.
A major problem with a window opening in the passivation layers is that moisture and contamination can enter through the exposed passivation layers and diffuse to the semiconductor devices. The diffused moisture and contaminates can decrease device reliability and yields. Moreover, moisture is present in the air and sodium (Na+ ions) are plentiful in the environment.
As shown in FIG. 3, moisture and other contaminates can enter through the window 28, diffuse into spin on glass layer 34 and diffuse to the adjacent semiconductor devices 31, 40, 64. Water will attack the metal interconnects 70, and sodium and other contaminates can harm MOS devices 31, 64.
First, water can attack the metal interconnects 70, with the following reaction: EQU 3 H.sub.2 O+Al.fwdarw.Al(OH).sub.3 +3/2 H.sub.2
The formation of Al(OH).sub.3 causes the resistance of metal interconnects 70 to increase and finally causes circuit failure.
Second, contamination can harm MOS devices. FIG. 3 shows an opening 28, buried N+ regions 60, 61, 62, field oxide 31, gate oxide 64, polysilicon gate 66, and metal layers 68, 70. Mobile ions, such as sodium ions, can diffuse through inter-metal dielectric layer 34, and through the insulating layer 34 into the field oxide layer 31. Mobile ions in the field oxide layer 31 can cause field inversion which allows an undesired leakage current between adjacent buried N+ (or P+) regions 60, 61 resulting in circuit failure. Also, mobile ions in the gate oxide 64 will cause a transistor threshold shift whereby the circuit fails.
The following three U.S. Patents show fusible link structures, but do not adequately solve the problem of contamination diffusing through the window opening to adjacent devices and not interfering with the laser trimming operation.
Billig et all, U.S. Pat. No. 5,025,300 teaches an integrated circuit where a very thin protective dielectric layer composed of silicon dioxide, silicon nitride or silicon oxynitride, is formed over the fuse in a window (Billig, Col. 4, lines 58-61). The dielectric layer has a thickness that allows the laser to penetrate, but the layer reduces the laser energy to the fuse, potentially preventing the laser from blowing the fuse. Also, because the dielectric layer interferes with the laser irradiation, its thickness uniformity must be tightly controlled to allow consistent laser cutting from fuse to fuse. In another embodiment of Billig, a thin protective dielectric layer is formed after the fuse is blown. This invention prevents shorts between conductors that otherwise might occur due to debris from the fuse-blowing operation. This embodiment has tradeoffs since the extra protective layer must be removed from some device surfaces, for example, over metal bonding areas. Also, forming the extra layer and partial removal steps add cost.
Takayama, U.S. Pat. No. 4,536,949 discloses a method of forming an opening over fuses where each insulation layer is etched separately yielding a more accurate opening. However, the problem of contamination diffusing to the devices through the exposed sidewalls of the opening still exists.
Motonami, U.S. Pat. No. 5,241,212, discloses a protective layer covering an opening through a single insulating layer for a fuse. In one embodiment, the protective layer only covers the surface of the protective layer, but leaves the sidewall of the insulating layer exposed for contamination to diffuse through. In a second embodiment, the protective coating covers the top of the isolation layer, the sidewalls of the fuse opening and also covers the isolation over the fuse and the fuse. Unfortunately, the protective layer over the fuse inhibits the laser from heating the fuse which can cause problems in consistently laser trimming (blowing) the fuse.
Thus, it is desirable to find a solution that prevents contamination from entering into the sidewalls of the openings, but does not interfere with the laser irradiation which heats the fuse in the laser trimming operation.